The invention relates to glazings of tempered glass, especially, but not exclusively,
1. Field of the Invention
Glazings of thin tempered glass (normally tempered float glass) for automotive use, a method of tempering a glazing, and to a novel glass composition suitable for use in the tempered glazings of the invention and in the method of the invention.
2. Discussion of Related Art
Prior to the 1970""s, automotive glazings were generally 4 mm or 5 mm thick or even thicker. The first oil crisis of the 1970""s encouraged a move towards thinner glazings for automotive use, especially in Europe and Japan, and towards solving the problems encountered in producing thin tempered glazings having the fracture characteristics required to meet official standards. In order to meet European standards, it was found necessary (because of the fracture characteristics of the glass) to provide a higher tempering stress together with an appropriate stress distribution (see, for example, UK patents GB 1 512 163 and GB 2 000 117) in order to achieve the required fracture patterns on breakage. Moreover, because of the reduced thickness of the glass, it was more difficult to achieve the temperature differential between the surface and core of the glass required to produce a given tempering stress. While satisfactory tempering was achieved in thicknesses of about 3 mm, the difficulties of tempering thinner glasses by conventional processes have inhibited progress in reducing glass thickness further so that, about 25 years after the introduction of such thin tempered automotive glazings, the commercial production of tempered automotive glazings having a thickness of less than 3.1 mm remains difficult.
We have now found that glazings, especially but not exclusively thinner glazings, can be more readily tempered including tempered to meet glazing standards (e.g. such as European automotive glazing standards) if the glass composition is appropriately modified, especially if the glass composition is modified to significantly increase its coefficient of thermal expansion and/or reduce its Fracture Toughness.
Certain selected glass compositions have previously been proposed for use in thin automotive glazing. International Patent Application WO 96/28394 relates to glass sheets of thickness in the range 2 to 3 mm having a total iron content (measured as Fe2O3) of 0.85 to 2% by weight, and specified optical properties, including a visible light transmission of greater than 70% and a total energy transmission of less than 50%. The glasses specifically described have a high alkali metal oxide content (ranging from 14.4% to 15.8% by weight) a magnesium oxide content ranging from 0.25% to 3.8% by weight and a calcium oxide content ranging from 8.4% to 8.6% by weight. The specification refers to the possibility of tempering single sheets of such thin glass for use in automotive side glazings, but make no reference to the difficulty of achieving commercially satisfactory tempering in practice.
International Patent Application WO 99/44952 relates to a sheet of soda lime silica glass designed to be heat tempered and characterised by a very high coefficient xcex1 of thermal expansion greater that 100xc3x9710xe2x88x927 Kxe2x88x921 (although it does not specify the range of temperatures over which xcex1 is to be measured), a Young""s Modulus E higher than 60 Gpa and a thermal conductivity K less than 0.9 Wmxe2x88x921Kxe2x88x921. The invention is said to make possible glass sheets of thickness lower than 2.5 mm which can be tempered to the requirements of ECE Regulation R43 using apparatus previously envisaged for the tempering of 3.15 mm glass. The particular glasses described all have a very high alkali metal oxide content (in the range 19.9 to 22.3% by weight) resulting in low durability and making the glasses expensive to produce.
According to the present invention there is provided a thermally tempered glazing of soda lime silica glass produced by tempering a pane of glass having a coefficient of thermal expansion, xcex1, greater than 93xc3x9710xe2x88x927xc2x0 C.xe2x88x921 and/or a Fracture Toughness, FT, of less than 0.72 MPamxc2xd. The invention is especially, but not exclusively, applicable to tempered glass panes less than 3 mm thick and to the tempering of such panes.
For the purpose of the present specification and claims, xcex1 is the value of the coefficient of thermal expansion per degree Centigrade of the glass measured over the range 100xc2x0 C. to 300xc2x0 C.; it is measured in accordance with ASTM E228 at constant heating rate. Preferably the coefficient of thermal expansion is at least 95xc3x9710xe2x88x927 per degree Centigrade, although modification of the composition to achieve a coefficient of thermal expansion greater than or equal to 100xc3x9710xe2x88x927, while beneficial to assist tempering, will generally be avoided on cost and durability grounds.
Toughness is the energy per unit area (Joules per square metre) required to make a crack grow. Fracture Toughness, FT, is related to Young""s modulus and surface energy by
FT=(2xc3x97Surface Energyxc3x97Young""s Modulus/1xe2x88x92v2)xc2xd
where v is Poisson""s ratio. For the purpose of the present specification and claims, it is determined by indenting a bar of glass using a Vickers indenter at a load sufficient to produce cracks at the corners of the indentation, and then breaking the bar in a 3- or 4-point bend test and the determining fracture stress, "sgr"f, in Pascals required for breakage. The Fracture Toughness of the glass, assuming it is in the fully annealed state*, is then given by
FT=xcex7(E/H)xe2x85x9"sgr"fxc2xePxc2xc
where xcex7 is a constant, E is Young""s modulus, H is the hardness of the glass, and P is the load used to create the indentation.
Flat glass will be the fully annealed state if it has been heated at the annealing temperature for one hour and cooled at 2xc2x0 C. per minute to room temperature. 
The constant xcex7 is determined with reference to FIG. 8.20 in Fracture of Brittle Solids (Brian Lawn, Cambridge University Press 1993). Applying values of E=70 GPa, H=5.5 GPa and FT=0.75 MPam{fraction (1/2 )}for soda lime silica glass the value of xcex7 is xcex7=0.44.
If the glass is not in the fully annealed state*, it is necessary to apply a correction for residual stress to the Fracture Toughness calculated using the above equation. In practice, it is convenient to measure the Fracture Toughness of glass in the fully annealed state.
Preferably, the glass has a Fracture Toughness of less than or equal to 0.70 MPamxc2xd, especially less than or equal to 0.68 MPamxc2xd.
In a preferred embodiment of the invention, the glass has a coefficient of thermal expansion xcex1 (xc2x0 C.xe2x88x921 in the range 100xc2x0 to 300xc2x0 C.) and a Fracture Toughness, FT (in MPamxc2xd) such that             α      xc3x97              10        7              FT    ≥  135
preferably xe2x89xa7140, and especially xe2x89xa7145.
It has been found that an increase in the alkali metal oxide content of the glass tends to increase the coefficient of thermal expansion, and while it is well known that glass can be produced with high alkali metal contents (patents relating to glass compositions for production by the float process typically propose an alkali metal oxide content in a range up to about 20%), an increase in alkali metal oxide content generally increases the cost of the glass and reduces its durability. In consequence, commercially available float glass generally has an alkali metal oxide content in the range of 13 to 14% by weight, and glasses with higher alkali metal oxide contents are not used in the production of thermally tempered glazings, especially automotive glazings. We have found that increasing the alkali metal oxide content by a relatively small amount results in a surprising increase in the ease of tempering (as measured, for example, by the particle count on fracture) of the glass (especially when associated with a modification of the alkaline earth metal oxide content of the base glass as explained below). Thus certain preferred glasses have an alkali metal content greater than 15% by weight, preferably less than 19% (to avoid excessive cost and loss of durability) and especially in the range 15% to 18% by weight, especially preferred glasses have an alkali metal content of 15% to 17% by weight. The sodium oxide content is preferably greater than 14.5% by weight.
Further improvements in ease of tempering appear to result from increasing the ferrous oxide content of the glass, and we especially prefer to use glass compositions containing at least 0.2%, especially at least 0.3%, by weight of ferrous oxide (calculated as ferric oxide), and, in one embodiment of the invention, that at least 30% (preferably at least 35%) of any iron oxide present to be in the form of ferrous oxide (where, in calculating the percentages, both ferric oxide and ferrous oxide are calculated as if ferric oxide).
The alkali metal oxide is believed to operate both by increasing the coefficient of thermal expansion of the glass (so increasing the stress differential between the surface layers of the glass and the core resulting from a given temperature difference between surface and core) and reducing the thermal conductivity of the glass (so increasing the temperature differential between surface and core when the surface is rapidly cooled in a thermal tempering process). However, the results achieved, especially with glasses containing significant amounts of ferrous iron, show a much greater increase in ease of tempering to meet European automotive glazing standards than can be accounted for by these effects alone, and these can be attributed, at least in part, to a reduction in Fracture Toughness of the glass.
One effect of increasing the alkali metal oxide content in a soda-lime-silica glass is believed to be an increase in the proportion of non-bridging oxygens (a bridging oxygen being an oxygen bonded directly to two silicon atoms. Sixe2x80x94Oxe2x80x94Si) present:
xe2x89xa1Sixe2x80x94Oxe2x80x94Si+Na2Oxe2x86x92xe2x89xa1Sixe2x80x94Oxe2x80x94Na+Naxe2x80x94Oxe2x80x94Sixe2x89xa1
The formation of such non-bridging oxygens in a silica lattice leads to a weakening of the glass structure, which is associated with reduced Fracture Toughness, and we have found that the reduced Fracture Toughness is associated with increased ease of tempering. The effect of incorporating alkaline earth metal ions in a silica lattice is similarly to displace oxygens directly bridging between silica atoms:
xe2x89xa1Sixe2x80x94Oxe2x80x94Sixe2x89xa1+MOxe2x86x92xe2x89xa1Sixe2x80x94Oxe2x80x94Mxe2x80x94Oxe2x80x94Sixe2x89xa1
where M is an alkaline earth metal. Differences in bonding strengths occur through different sizes of alkaline earth metal ions. In general, we believe the smaller the alkaline earth metal incorporated, the stronger the lattice and the higher the Fracture Toughness of the glass, with the difference between calcium ions and magnesium ions being particularly marked. Thus, to decrease the Fracture Toughness of the glass, it is desirable to maintain the magnesium content of the glass low (less than 2%, preferably less than 1%, especially less than 0.5%, all by weight), while avoiding use of an excessively high (from a cost viewpoint) proportion of alkali metal oxide will generally imply a content of alkaline earth metal oxide, other than magnesium oxide, of at least 9% and preferably at least 10%, by weight. Preferably, the glass will contain at least 9% and especially at least 10% of calcium oxide, and the total alkaline earth metal oxide content (including magnesium oxide) of the glass will normally be more than 10% by weight.
The glass will usually be float glass with a composition (in percentages by weight) of:
Certain of the glass compositions which may be used in the practice of the present invention are new, and according to a further aspect of the invention, there is provided a novel soda lime silica glass in sheet form of composition comprising in percentage by weight:
and any small proportions of additional components, for example, titania and other colouring agents, for example, selenium, cobalt oxide, nickel oxide, chromium oxide, cerium oxide.
Preferably the glass composition contains, in percentages by weight, 67-73% SiO2, 0-3% Al2O3, 0-3% B2O3, alkaline earth metal oxide (other than MgO) 10-14%, alkali metal oxide 15-17%.
While magnesium oxide contents below 0.5% may be preferred for optimum results, in practice, achieving a very low magnesium content will generally imply a long changeover time when the glass is made successively with a conventional glass containing a higher proportion (typically around 4%) of magnesium oxide and, in practice therefore, we normally prefer to employ glasses containing at least 0.5% by weight of magnesium oxide. Moreover, for such practical reasons, a magnesium oxide content in the range 0.75% to 1.5% by weight will commonly be preferred.
The novel glasses of the present invention will normally contain iron, either to modify the optical properties and/or enhance the temperability of the glass, or at least as an impurity (since the use of iron free batch materials is likely to add significantly to the cost of the batch); in the latter case it will normally be present in an amount of at least 0.05% by weight (calculated as ferric oxide).
In the former case, iron will normally be present in an amount (calculated as ferric oxide) of at least 0.5% by weight. For a particularly high performance i.e. high visible light transmission with relatively low solar energy transmission, the percentage of iron in the ferrous state will be above 30%. In other cases, the percentage of iron in the ferrous state will be less than 30% (i.e. the ratio of ferrous iron (calculated as ferric oxide) to total iron (calculated as ferric oxide) in the glass will be less than 30%).
Preferred ranges of compositions are as discussed above in relation to the tempered glazing of the invention. These glasses are used in sheet form and will normally have a thickness in the range 1 to 6 mm, especially 2 to 5 mm, and be formed by the float process.
A particularly preferred glass according to the present invention has the following composition in percentages by weight:
which composition is hereinafter referred to as Composition I. Composition I has a coefficient of thermal expansion, xcex1, of 98.9xc3x9710xe2x88x927xc2x0 C.xe2x88x921 (in the range 100xc2x0 C. to 300xc2x0 C.), and a Fracture Toughness of 0.66xc2x10.02 MPamxc2xd, so that, for Composition I:             α      xc3x97              10        7              FT    =            98.9      0.66        =    150  
The use of the specially selected glass compositions in accordance with the present invention facilitates the production of thin (less than 3 mm) tempered glasses, and is especially valuable in permitting the commercial production of tempered automotive glazings in thicknesses of 2.3 to 3 mm, especially 2.6 to 2.9 mm, by conventional tempering methods. It is known that glazings below 3 mm can be tempered using specialist tempering processes, such as powder tempering, or special tempering boxes available in commerce from Glasstech Inc of Perrysburg, Ohio, USA; it is the ability to temper the glass by conventional methods with satisfactory yields without additional cost that is especially valuable. The glazings may be tempered to meet national and international (especially European Standard ECE R43) standards for automotive glazings, especially sidelights and backlights.
Even thinner glasses, for example glasses having a thickness in the range 1.0 mm to 2.5 mm, especially 1.6 mm to 1.9 mm, can be semi-tempered e.g. tempered to semi-dicing fracture, e.g a surface compressive stress of at least 35 MPa, in accordance with the invention for use in laminated automotive glazings (especially opening side glazings required to pass a door slam test).
While the main advantage of using the special glass compositions of the invention lies in tempering thin glasses by conventional methods, their use in thicker glasses is also valuable in enabling the required stresses to be achieved with lower heat transfer coefficients and hence lower blowing pressures, with a consequent reduction in the use of energy.
Thus, according to a further aspect of the present invention there is provided a method of tempering a glazing (especially an automotive glazing) composed of glass having a high (greater than 93xc3x9710xe2x88x927xc2x0 C.xe2x88x921) coefficient of thermal expansion and/or a low Fracture Toughness (less than 0.72 MPamxc2xd) by operating at quench pressure at least 10% less, normally more than 20% less and preferably at least 25% less, than the quench pressure required to temper a corresponding glazing of standard composition to the required standards. Under optimum circumstances, use of the present invention may permit achievement of required tempering standards at a quench pressure 40% or more less than the quench pressure required to toughen a corresponding glazing of standard composition to those standards. The required tempering standards vary from country to country but generally require achievement of dicing fracture. By xe2x80x9crequired standardsxe2x80x9d we mean the standards required by the authorities in the country in which the glazing is to be used. In Europe, this will generally be ECE R43 for automotive glazings.
The method of the present invention is especially applicable to glazings having a thickness in the range 3 mm to 5 mm glazings and will generally result in the use of blowing pressures of not more than 12.5 kPa (50 inches water gauge), preferably not more than 10 kPa (40 inches water gauge), especially not more than 7.5 kPa (30 inches water gauge) for 3 mm glass, to not more than 7.5 MPa (30 inches water gauge), preferably not more than 6 kPa (24 inches water gauge) for 4 mm glazing, and not more than 6 kPa (24 inches water gauge) preferably not more than 5 kPa (20 inches water gauge), for 5 mm glass. The values for blowing pressure quoted above are generally applicable with dwell times (the times between the leading edge of the glass exiting the heated zone and the trailing edge of the glass entering the quench) of around 5 or 6 seconds; it will be appreciated, however, that the lower the dwell time (for a given temperature at exit from the heating zone), the lower the blowing pressure required.
The method of the present invention offers a number of advantages. The use of lower quench pressure results in a saving of energy and reduces the risk of a visible orange peel effect on tempering. Moreover, since a lower quench pressure may be used, equipment (especially air blowers) and conditions may be used to temper glazings of the selected glass compositions of the invention which are thinner than the conventional glazings which can be satisfactorily tempered using that equipment and those conditions; thus, for example, equipment and conditions capable of tempering glazings of conventional composition having a of at least thickness 5 mm may be used to toughen glazings of glass having a modified composition as taught herein of lesser thickness e.g. 4 mm.
The expression xe2x80x9cstandard compositionxe2x80x9d is used herein to refer to a known iron containing glass used extensively for production of tempered 3.1 mm automotive glazings and having the following composition in percentages by weight:
The glass has a coefficient of thermal expansion, xcex1, of 92.4xc3x9710xe2x88x927 (in the range 100xc2x0 to 300xc2x0 C.), and a Fracture Toughness of 0.71 MPamxc2xd, so that, for this glass,             α      xc3x97              10        7              FT    =            92.4      0.71        =    130  
Samples of the glass, referred to as OPTIKOOL(trademark) 371, are available from Group Intellectual Property Department, Pilkington plc, St Helens. England.
The invention is illustrated but not limited by the following examples which describe the thermal tempering of automotive side glazings and components therefor in accordance with the invention.